1. Field of the Invention
The present invention relates to a solder bump transfer sheet, a method for producing the same, a method for fabricating a semiconductor device, and a method for fabricating a printed board. More particularly, the present invention relates to a method for producing a solder bump sheet used advantageously for connection of a semiconductor chip to a board of a package.
2. Description of Related Art
Solder bumps are used for connection between a semiconductor device and a circuit board and connection between a semiconductor chip and a circuit board in order to provide both electrical connection and mechanical connection.
The “semiconductor device” as used herein refers to a package of a semiconductor chip, including not only a package of a single semiconductor chip, but also a package of a plurality of semiconductor chips, such as a multichip module (MCM). The “circuit board” as used herein includes not only a printed board on which a semiconductor device is mounted, but also a wiring board of a package on which a bare chip is mounted by flip chip bonding. The present invention is particularly advantageous when used for formation of narrow-pitch solder bumps. Herein, therefore, solder bumps for flip chip bonding will be described.
Paste printing and ball mounting are conventionally employed as methods for forming solder bumps. However, in paste printing, formation of fine bumps is difficult. It is also difficult to control the height (thickness) of bumps with high precision. In ball mounting, comparatively fine bumps can be formed. However, an expensive mounter is necessary to mount and press solder balls shaped into a perfect sphere (diameter: 0.76 mm±0.02 mm, for example) at predetermined positions. This greatly increases production costs. Moreover, since handling of fine balls with reliability is difficult work, production efficiency decreases.
The present inventor disclosed a method for forming fine solder bumps using a solder bump transfer sheet in Japanese Laid-Open Patent Publication No. 11-312758, in which a film formed by plating is patterned by a photolithographic process, for example. By this method, fine bumps with a reduced variation in height (thickness) can be produced with great efficiency.
However, as a result of further investigation by the present inventor, it has been found that the solder bump transfer sheet disclosed in Japanese Laid-Open Patent Publication No. 11-312758 has a variation in the transferability, or the degree of transfer, of solder bumps. The disclosed transfer sheet has difficulty in transferring a large number of bumps, for example, more than 1000 bumps to a chip or the like with reliability, and this may cause decreases in the yield. This problem is more serious as the integrity of the semiconductor device enhances.
For solder bumps used for connection of a semiconductor chip having a large-capacity DRAM (for example, an ASIC), it is necessary to use a solder material of which the content of an α-ray source is small, to prevent the DRAM from malfunctioning due to an α-ray. Solder material costs for such a low α-ray are high (for example, three times as high as gold). Since the costs are high for this type of semiconductor chip itself, the overall production cost further increases. In consideration of this, improvement of the yield is strongly desired.